Method for passivation of strip black plate

ABSTRACT

A process for passivation of strip steel plate, having the following steps: electrochemical treatment of the black plate by passing the black plate through an electrolyte to form an inert steel surface; rinsing the black plate; and application of an aqueous chromium-free treatment solution to at least one surface of the black plate to form a conversion layer that protects against corrosion and an adhesion layer for paints and organic coating materials. The black plate treated in accordance with this process is characterized by high corrosion resistance and has good bonding capacity for paints and organic coatings and therefore is very suitable as a substitute for tin-free steel (TFS or ECCS) and tinplate for the production of packagings, in particular cans. In contrast to the traditional manufacturing and passivation processes for tin-free steel and tinplate, no chromium VI, which is environmentally hazardous and hazardous to health, is used in this process.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2013 107 506.1 filed 16 Jul. 2013, the entire contents of which are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The disclosure concerns a method for passivation of strip black plate, where in a first step, an inert layer is formed on the black plate surface by an electrochemical treatment of the black plate, and in an additional step, an aqueous chromium-free treatment solution is deposited on at least one surface of the black plate to form a corrosion-protective conversion layer, which additionally forms a bonding layer for paints and organic coating materials. Passivation here is understood to mean the targeted production of a protective layer (here: conversion layer) on the black plate that prevents or at least greatly slows the corrosion of the black plate. The disclosure additionally concerns the use of black plates treated in accordance with the disclosure as packaging steel.

There are known processes for protection of metal surfaces from corrosion, in which the metal surface is provided with a coating of another, as a rule non-noble, metal (such as zinc and chromium). For example, coating steel sheet with zinc or chromium or even with tin (which, to be sure, is a more noble metal than steel) is known. For example, tin-plated ultrafine plate (tinplate) is very widely used for production of packaging, in particular in the food area. Tinplate is characterized by very good corrosion resistance and good forming behavior as well as weldability and is therefore very well suited for production of packagings such as beverage cans.

To protect the metal coating, or the tin coating in the case of tinplate, from corrosion and to produce a good bonding base for paints and plastic coatings, conversion layers are often applied to the surface of the metal coating.

Conversion layers are understood to be very thin nonmetallic, mostly inorganic layers on a metal surface, which as a rule are produced by the chemical reaction of an aqueous treatment solution with the metal substrate. Conversion coatings guarantee, in particular in the case of ultrafine steels, a very effective corrosion protection, a very good bonding base for paints and plastics, and they reduce surface friction and wear.

Depending on the substrate one distinguishes between iron, zinc, or manganese phosphating, electrolytic phosphating, or chromate, oxalate, and anodization processes. Chromium-containing conversion layers have proved to provide very effective corrosion protection. In a chromating process, the metal surface is treated with an acidic, chromium(VI) ion-containing solution, in which chromium(VI) is reduced to chromium(III). A chromium-containing conversion layer that protects against corrosion is formed on the metal surface by the treatment.

Chromium(VI) compounds are, however, acutely toxic and carcinogenic. Passivation of metal surfaces with chromium(VI)-containing substances has already been banned in the EU for applications in automobile manufacture and household appliances. For this reason, chromium-free conversion layers have been developed in the prior art. For example, processes for generation of chromium-free conversion layers on zinc and aluminum surfaces are known from WO 97/40208-A and EP 2532769 A1. Furthermore, treatment solutions for generation of chromium-free conversion layers that contain oxo cations and halogen complexes and that lead to colorless and slightly iridescent layers are described in WO 2008/119675.

Tinplate has outstanding properties as packaging material for foods and has been manufactured and processed for this purpose for many decades. However, tin, which is the corrosion-inhibiting coating in the case of tinplate, has become a relatively valuable material because of the global shortage of resources. As an alternative to tinplate, in particular for use as packaging steel, steels that have been electrolytically coated with chromium, which are called “tin-free steel” (TFS) or “electrolytic chromium coated steel” (ECCS), are known from the prior art. These tin-free steels are characterized on the one hand by good bonding capacity for paints or organic protective coatings (for example, of PP or PET), but on the other hand have considerable disadvantages in the conduct of the coating process because of the toxic and health-threatening properties of the chromium VI-containing materials that are used for the coating.

SUMMARY OF THE DISCLOSURE

One goal of this disclosure therefore consists of making available a chromium-free packaging steel, which is suitable as a substitute for tin-free steel (TFS or ECCS) and as a substitute for tinplate and which in particular should be comparable to tinplate or tin-free steel both with regard to corrosion resistance and bonding capacity for paints or organic coatings.

According to the process as disclosed herein, an uncoated black plate in strip form is used and its surface is inertized in a first process step by an electrochemical treatment, then is rinsed with water or another rinse liquid, and finally in an additional step is coated with a corrosion-resistant conversion coating by depositing an aqueous chromium-free treatment solution onto at least one surface of the black plate.

Other special embodiments of the process are also disclosed herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

Cold-rolled, annealed, and rerolled or cold-finished steel strip made from an unalloyed steel with carbon content 20-1000 ppm is preferably used. The steel strip (black plate) preferably has the following properties:

-   -   Strength: 300-1000 MPa     -   Elongation at break: 1-40%     -   Thickness: 0.05-0.49 mm     -   Surface roughness: 0.1-1 μm.

The steel can be, for example, a ferritic steel or even a multiphase steel that has a plurality of structural components, in particular ferrite, martensite, bainite and/or residual austenite. Such multiphase steels are characterized by a high strength of more than 500 MPa, while at the same time having good elongation at break of more than 10%. In view of the intended use of the black plate treated in accordance with the disclosure as packaging steel, the steel grades defined in DIN EN 10202:2001: “Cold-rolled packaging steel products (electrolytically tinplated and chrome plated)” are used. Among other things, analysis and mechanical characteristics of the steel are defined in this standard. The qualities in particular lie between TS230 (soft bell furnace grade, tensile strength 230 MPa) to TH620 (DO, 620 MPa).

To conduct the process in accordance with the disclosure, the black plate, which is in strip form, is moved at a strip speed of preferably more than 200 m/min and up to 750 m/min. First there takes place a step to form an inert layer on the surface of the black plate through an electrochemical treatment. To prepare for the electrochemical treatment, the moving black steel plate is first cleaned and in particular degreased in an optionally necessary pretreatment step, after which it is rinsed, pickled, and again rinsed. This is optionally necessary, since the cold-rolled and recrystallization-annealed black plate as a rule is post rolled or dressed after the recrystallization annealing, where, for example, in wet rerolling with a water/oil suspension or even in dry rerolling, the black plate surface becomes contaminated by oil, abraded iron, soap, and other contaminants. This contamination is remedied by the (optional) pretreatment step.

For this, the black plate is guided into a cleaning tank containing an alkaline sodium or potassium hydroxide solution. The concentration of the alkaline degreasing agent is preferably between 20 and 100 g/L at a bath temperature of 20-70° C. A degreasing of the black plate expediently takes place in two steps, where an immersion process is carried out in a first step and an electrolytic process with current densities of 2-30 A/dm² is carried out in a second step. After the degreasing, both sides of the black plate strip are rinsed, for example, by a triple cascade rinse with 10-30 m³/h in each cascade. If necessary, oxide residues can be removed by guiding the black plate strip into additional cleaning tanks containing a salt or sulfuric acid pickling solution having a concentration of, for example, 10 to 120 g/L in two successive immersion operations, followed by an immersion rinse with an immersion dip. The temperatures of the pickling solution and the rinse water typically lie between 20 and 60° C.

After the pretreatment, a homogeneous, solid, inert steel surface is produced by means of electrochemical treatment by passing the black plate strip through an electrolyte. The electrolyte is preferably alkaline. The electrochemical, preferably alkaline, treatment of the black plate serves for inertization and for leveling out the surface properties of the steel strip before application of the conversion coating.

In the electrochemical treatment process step, the black plate strip is guided at the strip speed through an electrolyte bath while connected as the anode at a preferred current density of 2-30 A/dm². The electrolyte is, for example, a sodium hydroxide a sodium hydroxide solution with a preferred NaOH concentration of 20-100 g/L, and in particular a sodium hydroxide bath containing a 3% NaOH solution. For example, a soda solution, in particular a 5% sodium carbonate solution (Na₂CO₃), can also be used as electrolyte. The bath temperatures of the electrolysis bath are expediently kept between 20 and 80° C.

After the electrochemical treatment, the black plate is rinsed with water. The rinse can take place by immersing the strip in a water tank or by spraying with water. Demineralized water (VE), deionized water, osmosis water, or distilled water with temperatures of 20-60° C. is preferably used for this. However, untreated potable water or other rinse liquids can also be used. The black plate is dried after being rinsed.

Finally, in a concluding step, a conversion layer is deposited on at least one surface of the black plate by applying an aqueous chromium-free treatment solution to the surface of the black plate that was inertized beforehand in the electrochemical treatment.

The conversion layer is expediently deposited in a no-rinse process, i.e., a rinsing is omitted after generation of the conversion layer. The aqueous chromium-free treatment solution that forms the conversion coating is applied, for example, to the surface of the black plate, for example, with an application device that comprises a roll coater, a rotary sprayer, or spray nozzles.

An application device with a rotary sprayer is preferably used for application of the aqueous treatment solution. Before application of the treatment solution, the surface of the black plate to which the conversion layer is to be applied should be as clean and dry as possible. For this reason, at least the surface of the black plate that is being coated with the conversion layer is dried with a drying device, for example an air knife. With this air knife, a laminar hot air stream is blown onto the surface of the moving strip, so that problematic foreign particles are blown off of the steel strip surface and the steel strip surface is dried.

The rotary sprayer has a plurality of spray rotors arranged side by side across the strip direction to which the aqueous treatment solution is supplied and which are set into rotation by a drive in order to spray the aqueous treatment solution by centrifugal force in the form of a fine spray jet onto the one or both surfaces of the strip and to form a wet film of the aqueous solution there. After application of the wet film of aqueous treatment solution, it is leveled on the surface of the black plate by means of driven smoothing rollers. The smoothing rollers are expediently arranged in reference to the black plate surface(s) so that they exert only a little pressure on the wet film of aqueous treatment solution and do not squeeze out any, or at most a minimal fraction, of the applied treatment solution from the surface. The amount of treatment solutions sprayed by the rotary sprayer is appropriately adjusted so that an excess does not remain on the black plate surface. In this way, an otherwise necessary disposal or processing of the excess amount of treatment solution is no longer necessary. After leveling, the sprayed wet film it is dried so that a dry weight of the treatment substance remains on the treated surface or surfaces. Expediently, the dry weight of the treatment solution after drying is between 1 and 50 mg/m² and preferably lies in the range of 10 to 30 mg/m². The amount of the aqueous treatment solution delivered to the spray rotors of the rotary sprayer per unit of time is expediently matched to the strip speed. Through this, it can be guaranteed that only the precisely required amount of fresh treatment solution in the appropriate concentration is applied as a wet film to the black plate strip by the rotary sprayer. In this way, for example, a constant application weight of wet film in the range of 2 mL/m² to 8 mL/m² and preferably about 5 mL/m² per strip side can be established independent of the strip speed.

After the application of the wet film of the treatment solution, the strip is sent through a strip dryer in order to dry the wet film. After drying, there remains on the surface of the black plate, per side, a dry application weight of the thus formed conversion layer of 2 mg/m² to 30 mg/m².

The desired dry weight of the conversion layer can be adjusted through the amount of treatment solution delivered to the rotary sprayer per unit of time.

An advantage of this kind of application lies in the fact that only fresh treatment solution is always used and it cannot be contaminated by stripped iron through contact and recycling with the steel strip. In addition, it turned out that the process is very economical, since only the precisely required amount is applied and an excess is not required, so that excess treatment solution no longer needs to be collected. This can prevent the formation of wastewater that needs to be treated later.

Alternatively, the application of the treatment solution can also take place via roll coaters through a roller application, likewise on a predried black plate surface. Roll coaters are preferably used in the lower strip speed range, and in particular at strip speeds of less than 200 m/min. Alternatively, the application can also take place by spraying the treatment solution or by immersing the strip in a bath containing the treatment solution. Since in this case the treatment solution is applied to the black plate in an excess amount, to achieve a predetermined desired application weight of the conversion layer, it is necessary to squeeze off the excess amount of the wet film, for example by means of squeeze rolls, where this can take place “wet in wet.” In this process, however, the solution is not applied in a constant uniform way independent of strip speed, and the treatment solution can, moreover, become contaminated by iron and then must be refreshed and, after exceeding a contamination threshold, be disposed of.

Finally, the wet film of treatment solution applied with the described application process is dried to form a dry conversion layer. This can take place, for example, by passing the black plate through a drying oven in which the wet film is dried by hot air or IR radiation. The drying preferably takes place at temperatures of 50-250° C. Then the surface of the dried conversion layer is lubricated or after treated with dioctyl sebacate (DOS), acetyl tributyl citrate (ATBC), butyl stearate (BSO), or polyalkylene glycol, in particular polyethylene glycol (PEG, preferably with a molecular weight of 6000 g/mol), or a combination thereof. An after treatment by lubrication with DOS, ATBC, BSO, or PEG expediently takes place electrostatically, with commercial lubricators as in the case of ECCS or tinplate, or also by means of a rotary sprayer.

The treatment solution used for the conversion coating preferably contains at least one of the following substances:

-   -   metal components: chosen from Ti, Zr, Mn, Zn, P, and         combinations thereof;     -   organic components: chosen from polyacrylate, polycarboxylate         and combinations thereof.

Metal and organic components can in turn be combined.

The treatment solution additionally contains at least one bonding agent for paints or organic coating materials, where the bonding agent in particular contains components of maleic acid, isophthalic acid, and cyclohexanedimethanol (CHDM), or combinations thereof. Compositions that contain polyethylene terephthalate (PET) or polycyclohexylenedimethylene terephthalate (PCT), such as glycol-modified polyethylene terephthalate (PET-G, which contains less than about 30% CHDM) or PCTG (which contains more than about 30% CHDM), proved to be especially suitable bonding agents.

The application amounts are 1 to 50 mg/m² for the relevant substances.

Some selected commercially available agents that are suitable for generation of conversion coatings using the process in accordance with the disclosure are listed below.

Important chemical Manufacturer/ Trade name component Distributor — Polycarboxylates BASF ® EFKA ® 4560 Modified polyacrylates BASF ® — Ti, Zn, Mn phosphate Henkel ® Granodine ® 1456 Ti, Zr Henkel ® GTP ® 10861 Zn phosphate, Ti Chemetall ® Gardo ® TP GB X4744A Mn, Ti, Zr Chemetall ® Gardobond ® (Chemetall ®) GB X4591 A1 Ti, Zr Chemetall ® GB X4744 Ti, Zr Chemetall ®

Preferred treatment solutions for generation of the conversion layers can, for example, be composed as follows:

a) an aqueous solution that contains aluminum fluorozirconate having a mol ratio of Al:Zr:F of (0.15 to 0.67):1:(5 to 7), where the total concentration of Al+Zr+F is 0.1 to 2.0 g/L and the pH is adjusted to below 5, preferably 3 to 5.

b) an aqueous solution that essentially contains:

-   -   0.2 up to less than 10 g/L zinc ions,     -   0.5 to 25 g/L manganese ions, and     -   2 to 300 g/L phosphate ions, calculated as P₂O₅,

where the zinc:manganese weight ratio of the phosphating solution is maintained in the range of 0.05:1 to 1:1.

c) an aqueous solution that contains zinc and manganese, with zinc in the range of 0.05 to 5 g/L, manganese in the range of 0.075 to 5.2 g/L, and copper in the range of 0.008 to 0.05 g/L, and/or a total of 0.002 to 0.5 g/L hexafluoride complexes of boron, aluminum, titanium, and/or zirconium, calculated as F₆.

d) an aqueous solution containing at least one film-forming agent, which contains at least one water-soluble or water-dispersed polymer having an acid number in the range of 5 to 200, and at least one inorganic compound in particulate form with an average particle diameter, measured on a scanning electron microscope, in the range of 0.005 to 0.3 μm diameter, where the polymer is selected from at least one plastic resin based on acrylate, ethylene, polyesters, polyurethane, silicone polyesters, epoxide, phenol, styrene, urea formaldehyde, their derivatives, copolymers, polymers, mixtures, and/or mixed polymers, and the inorganic compound in particulate form is selected from at least one compound of aluminum, silicon, titanium, zinc, and/or zirconium.

or

e) an aqueous solution containing:

-   -   i) at least one organic film forming agent, which contains at         least one water-soluble or water-dispersed polymer, which is a         plastic resin based on polyacrylic acid, polyacrylate, and/or         polyethylene acrylic acid, or a plastic resin mixture, and/or a         mixed polymer containing a plastic resin based on acrylate or         polyacryl, and     -   ii) a content of cations and/or hexa- or tetrafluoro complexes         of cations selected from the group consisting of titanium,         zirconium, silicon, aluminum, and boron, in the range of 0.2 to         30 g/L with respect to the content of the elemental metal.

The process in accordance with the disclosure can be integrated into an existing coating plant, for example in a strip coating plant for production of ECCS (or TFS), without great installation costs. The strip speed in such strip coating plants typically is 80-600 m/min.

The process in accordance with the disclosure has the advantages of a chromium-free and thus environmentally friendly and not health-hazardous, as well as low-cost coating on traditional black plate, in particular in the fine and ultrafine steel thickness range. Moreover, an effective savings of costs and energy is achieved through the selected application process (no-rinse process) for application of the conversion layer due to the omission of the final rinse. Additional advantages are achieved in particular through the combination of inorganic passivation and polymer-containing thin film coating. The black plates treated with the process in accordance with the disclosure are outstandingly suitable for production of packaging, in particular cans, and therefore can replace the tinplate and tin-free steel (TFS or ECCS) that are traditionally used as packaging steel. With regard to their corrosion resistance, these black plates are comparable to tinplate and have good adhesion properties for paints and plastic coatings, for example of PP or PET, that are comparable to tin-free steel (TFS or ECCS). 

What is claimed is:
 1. Process for passivation of strip black plate, having the following steps: electrochemical treatment of the black plate by passing the black plate through an electrolyte to form an inert steel surface, rinsing the black plate, application of an aqueous chromium-free treatment solution to at least one surface of the black plate to form a conversion layer that protects against corrosion and a bonding layer for paints and organic coating materials.
 2. Process as in claim 1, in which before the electrochemical treatment, the black plate is first degreased, then rinsed, pickled, and again rinsed.
 3. Process as in claim 1, in which the black plate is dried after the application of the treatment solution in order to form a dry application weight of the treatment solution on the black plate surface.
 4. Process as in claim 1, in which the electrochemical treatment takes place by guiding the black plate through an alkaline electrolyte while connecting the black plate as anode.
 5. Process as in claim 1, in which the conversion layer is deposited by applying the aqueous treatment solution in a no-rinse process.
 6. Process as in claim 5, in which the aqueous treatment solution is either applied to the black plate without an excess by means of a rotary sprayer or is applied in an excess amount with a roll coater or spray nozzles and then the excess treatment solution is squeezed off with squeeze rolls.
 7. Process as in claim 3, in which the drying takes place at 50-250° C.
 8. Process as in claim 3, in which the dry applied amount of the treatment solution is subsequently treated with DOS, ATBC, BSO, or a polyalkylene glycol, in particular polyethylene glycol (PEG).
 9. Process as in claim 3, in which the dry weight of the treatment solution is between 1 mg/m² and 50 mg/m².
 10. Process as in claim 1, in which the black plate is moved at a strip speed of at least 200 m/min.
 11. Process as in claim 1, in which the black plate is a cold-rolled, annealed, and rerolled, in particular ferritic or multiphase, steel with a carbon content of 20 to 1000 ppm.
 12. Process as in claim 1, in which the temperature of the electrolyte in the electrochemical treatment of the black plate is between 20 and 50° C.
 13. Process as in claim 1, in which the current density in the electrochemical treatment of the black plate is between 2 and 10 A/dm².
 14. Process as in claim 1, in which the aqueous treatment solution is free of chromium and contains at least one of the following components: metal components: selected from Ti, Zr, Mn, Zn, P, and combinations thereof; organic components: selected from polyacrylate, polycarboxylate, and combinations thereof
 15. Process as in claim 1, wherein the treatment solution contains at least one bonding agent for paints or organic coating materials, in which the bonding agent in particular has components of maleic acid, isophthalic acid, and cyclohexanedimethanol (CHDM), or combinations thereof.
 16. A packaging steel for production of packagings, in particular cans, comprising a black plate treated by a process in accordance with claim 1, with a chromium-free conversion layer on at least one surface of the black plate. 